The present invention relates to paper manufacturing systems and pulp drying manufacturing systems for controlling press section dewatering via the metered application of chemical dewatering agents and thereby having a positive effect on various sheet characteristics imparted by the press section consolidation process.
During a papermaking process in a typical paper machine, a furnish of fiber and water is fed onto a traveling forming fabric. Most of the water then drains through the fabric, to form on the fabric a fibrous web or mat of fibers that includes paper fibers from the furnish. On most paper machines, the paper web exits the couch containing 75 to 80% moisture.
The wet web travels from the couch into a press section where the moisture content can be mechanically reduced to 45-60%. The press section of a paper machine utilizes hydraulic pressure through a series of press nips to subject the web to compressive forces to remove as much water from the web as possible before the sheet continues on to the dryer section. The press section also consolidates the sheet to improve sheet strength, reduce bulk, increase sheet smoothness, and ensure uniform cross direction (CD) moisture distribution.
The web then travels to a drying section in which it traverses drying drums that reduce the water content of the web through evaporation to a final desirable level, yielding a paper product that can be cut or otherwise processed and packaged. Typically, the dryer section produces a paper sheet containing 5-10% moisture.
The extent to which the press section removes water from the web before the dryer is of prime importance to achieving efficient and economical paper machine operation because the drying sections consume large amounts of energy. The dryer section uses steam heat to evaporate free and bound water from the sheet and is the most expensive part of a paper machine in terms of capital and operating costs. Although only 1% of the water in the furnish is removed in the dryer section, the cost per unit of water removed is greater than 20 times that by the press section.
Steam consumption in the dryer section increases dramatically with increased moisture content of an incoming sheet. For example, for a 7% increase in sheet moisture entering the dryer section, steam usage increases by 34% to obtain the same moisture level in the dried sheet. Moreover, the latent heat of steam decreases as its pressure increases making it more costly to operate dryers at high steam pressures even though the steam temperature is higher. Consequently, it is desirable to maximize the removal of water from the web before it enters this section.
As the wet web traverses the press section it is in contact with one or more press fabrics or belts, where the latter can be also defined as a type of press fabric for the purposes of this discussion. Pressing of the web is done between two rolls in the press nip. As the web enters the nip, compression of the web and press fabric begins with entrained air flowing out of both the web and fabric. As the hydraulic pressure increases, water moves from the web into the fabric. When the fabric becomes saturated, surplus water flows out of the fabric. At this point the web is at the point of closest separation between the rolls and the hydraulic pressure is at a maximum. As the web moves out from this point, the pressure returns to zero and the paper sheet is at its maximum dryness. Finally, the paper and fabric exit the press nip and separate from each other causing a slight vacuum in the paper which could result in some rewetting of the paper sheet.
This reabsorption of water is undesirable and efforts have been made to minimize this effect by press section suppliers and paper machine clothing manufacturers. On the machine side, rapid separation of sheet and fabric is made to reduce the rewetting time. In addition, various types of water receptacles are provided to help remove water from the fabric. Paper machine clothing manufacturers also use impermeable belts or low permeability fabrics in specific press positions to reduce the rewetting effect.
The operational factors that determine the amount of water released from the web in a press section can be divided into three categories including machine design, stock and sheet properties, and operational elements. Machine design factors are fixed by the equipment manufacturer and are not controlled by the press operator. These factors include roll hardness and diameter, press configuration, and press nip design.
Variations in the properties of the web entering the press section influence the moisture content of the sheet exiting the press section. These properties include the type of furnish, freeness, amount of fiber fines, amount of filler, inherent water retention, compressibility, basis weight, web temperature, and moisture level. During the papermaking process, these characteristics fluctuate to varying degrees in uncharacterized ways and cause variations in the ultimate moisture content of the sheet leaving the press section. Attempts are made to minimize these variations but are of limited success during paper production.
In the press section, operational factors, such as machine speed, press load, and press fabric design and maintenance can be manipulated to optimize press section efficiency. In practice, these factors are difficult to control since the degrees to which each factor affects sheet moisture at any point in time is generally unknown. Press fabric cleaning and service life have a substantial influence in overall press section operation and are given attention by operators at significant cost to the mill.
The factors described above act as process variances that affect the final product. Currently, few, if any, of these factors are measured during the papermaking operation.
In paper manufacturing it is desirable to maintain an even moisture distribution throughout the sheet as it forms in order to produce high quality paper with uniform basis weight distribution. Poor moisture distribution leads to localized over or under drying, inferior paper quality, increased machine operating costs, and reduced efficiency. Thus, during a run, web moisture before the press section, especially in the cross direction, would be potentially the most important parameter to measure and control. However, in the past this has not been done for reasons of cost and difficulty of implementation.
If moisture were measured, a feed forward control method for controlling various processing parameters could be developed to more accurately control the production of each grade and basis weight of paper. To accomplish this a moisture sensor could be used to determine the moisture of the web just before it enters the press section. A controller could use the moisture value to anticipate the expected moisture of the product exiting the press section and determine a control action to adjust a control element, if necessary. For example, press load, press vacuum or shower water temperature in the press section could be adjusted so as to drive the predicted moisture of the sheet exiting the press to a more desirable value.
The feed forward approach would be difficult to develop as it would require a comprehensive and quantitative knowledge of how press section operational parameters affect dewatering of paper webs having varying incoming moistures and would require quantitative knowledge of how variations in any other unmeasured disturbances, such as web properties and type of paper being made, are affected by such adjustments. Moreover, because feed forward control would only be applicable to the actual press equipment being controlled this information would be unique for each press assembly.
Another potential approach for process control would be to use a feedback loop, which would monitor a measured output variable such as sheet moisture exiting the press section. A feedback controller could then manipulate a process variable such as steam supply, if needed, so that the product has more desirable characteristics. The feedback control algorithm, though unaware of specific disturbances acting on the process, would be able to maintain the output at the desired value so long as the function that manipulates the value is valid and does not cause the process to run in a region outside that for which the control algorithm is defined.
This feedback method and several of its variations (proportional, proportional+integral, or proportional+integral+derivative) have been practiced in other industrial applications such as liquid level control and temperature control.
Feedback control has been implemented by some mills in which final sheet moisture is measured using moisture monitors such as gamma gauges or infrared monitors. These measurement devices are either stationary, in which the moisture content is taken at one location on the cross direction of the sheet, or moving, for which a moisture profile is obtained along the width of the paper. The moisture value is fed to a controller in real time and compared to a preferred moisture level of the final sheet. Based on the difference, a corrective control action is applied to an appropriate control element in an earlier stage of the papermaking process, such as the degree of refining or dilution of the headbox stream, to obtain more desirable final sheet properties.
However, before implementing a process for controlling press section dewatering, detailed studies are required to define the relationships between control actions and their effect on press section dewatering. For example, it would be necessary to determine that increasing the first nip pressure by 46 psi/percentage moisture, water removal increases from 59.2 to 62.8%, when the web has an initial moisture level of 77.1 to 79.2%, a basis weight range from 61 to 82 g/m2, sheet speeds are 805 to 1023 ft/min (240 to 310 m/min), for a light board furnish in the temperature range of 103 to 112° F. (39 to 45° C.). As can be appreciated, a suitable control algorithm would be complicated and time consuming to develop.
For feed forward and feedback strategies, a control action could be specified for a narrow set of operating conditions and output values. However, any uncharacterized disturbance could affect the accuracy of the control algorithm. Algorithm modifications would often be required for routine changes in paper grades or final sheet specifications. Press section operation on new grades of paper would also have to be characterized prior to the development of suitable algorithms. In addition, comparing feed forward control and feedback control, feedback control would be slower since it is dependent on process lag times. However, feedback control would potentially be more forgiving in situations where the process is not completely characterized.
New control systems and methods are needed that require only a basic understanding of the paper manufacturing process without detailed knowledge of the quantitative relationships between inputs and outputs. Ideally such methods could optimize dewatering on any press section, leading to increased paper or paperboard production, and the direct development of improved sheet characteristics despite unknown variations in paper web disturbances or machine types. Additionally, the result of improved sheet moisture control exiting the press can impact other press operational choices such as press fabric design, press load and roll cover characteristics such that these choices enable improved final sheet characteristics to be consistently achieved. Such methods could also be used to lower steam pressures, allowing for the use of cooler roll temperatures in the dryer section. Other advantages to the use of cooler roll temperatures include less radiation loss, lower risk of steam leakage, and in many cases, higher quality paper can be obtained from lower contact surface temperatures.